1. Field of the Invention
The present invention relates to an improved process for producing an organic carboxylic acid having (n+1) carbon atoms by reacting alcohol having n carbon atoms with carbon monoxide in the presence of a rhodium catalyst system. Particularly, the present invention relates to carbonylation of methanol catalyzed by a rhodium system to produce acetic acid.
2.Description of the Prior Art
It is a well known and commercialized technology for obtaining acetic acid by carbonylation of methanol with rhodium catalysts. Comparing with the early cobalt catalysts, the rhodium catalyst system has advantages of lower reaction temperature and lower carbon monoxide partial pressure as well as faster reaction rate. Typically, carbonylation of methanol with a rhodium catalyst system proceeds in a liquid phase comprising of a rhodium catalyst as dissolved therein and a promoter including methyl iodide. Related art in regard to this can be referred to the Applied Homogeneous Catalysis with Organometallic Compounds, Vol. 1, pp. 104-138 (1997).
A technology for producing acetic acid by carbonylation of methanol has been disclosed in U.S. Pat. Nos. 3,769,329 and 4,690,912 under reaction conditions as follows: reaction temperature of 180xc2x0 C., pressure of carbon monoxide between 35-70 Kg/cm2, rhodium as a catalyst and methyl iodide as a promoter. It is also disclosed in these patents that the most effective solvent for producing the acetic acid is the product acetic acid per se. The main advantages of such a catalyst system are the extremely high conversion rate and selectivity ( greater than 95%). The catalyst can substantially be recycled to the reactor except some minor loss resulting from the pipe line or the pump leakage. This art can be deemed almost perfect except that for avoiding the precipitation of the rhodium catalyst and for maintaining a fairly higher reaction rate, the water content in the reaction system may have to be maintained at least 14xcx9c15 wt %. In the teachings of Hjortkjaer (Ind. Eng. Chem. Prod. Res., 1976, 15, p46), increasing the water content from 0 to 14 wt % in such a catalyst system will increase the reaction rate of methanol carbonylation correspondingly while the reaction rate is unchanged as the water content is above 14 wt %. Such a high water content will increase the expense for the separation equipment and will consume considerable energy. In recent decades, variable methods have been suggested in many patents one after another intending to enhance the solubility of the rhodium catalyst at lower water content( less than 14 wt %) .
European Patent Publication No. 0055618 has disclosed a method for carbonylation of methanol wherein the precipitation of rhodium catalyst at lower water content is alleviated by adding organic stabilizer for the catalyst. The stabilizer as disclosed therein includes several kinds of organic compounds, alone or simultaneously, containing one or more nitrogen atoms, phosphorus atoms or carboxyl groups selected from:
(1) N, N, N1, N1-tetramethyl-O-phenylenediamine) and 2,31-dipyridyl;
(2) HOOCxe2x80x94Y1xe2x80x94COOH and (HOOCxe2x80x94Y2)(HOOCxe2x80x94Y3)Nxe2x80x94Y1xe2x80x94N(Y4xe2x80x94COOH)(Y5xe2x80x94COOH),Y1-5=(CH2)m;
(3) (R1)(R2)Pxe2x80x94R3xe2x80x94P(R4)(R5), R1-5=alkyl group;
U.S. Pat. No. 4,733,006 has disclosed a method of utilizing an inorganic salt additive XOAc (X=Li+, Na+, Ka+) to alleviate the precipitation of rhodium catalyst in the reaction solution for methanol carbonylation at lower water content. However, no description regarding the influence of the inorganic salt additive to the reaction rate is mentioned through the whole patent.
It is disclosed in U.S. Pat. No. 5,001,259 that when utilizing inorganic iodide such as lithium iodide (LiI) as a stabilizer for rhodium catalyst to alleviate the precipitation of rhodium at lower water content during the carbonylation of methanol, reaction rate which is approximately comparable to the high water content (14 wt %) can be obtained. In the same patent, a kind of quaternary ammonium salt, N-methyl-picolinium iodide, is also disclosed for raising the carbonylation rate at lower water content. Unfortunately from the experiment result, it is found that the compound N-methyl-picolinium iodide forms a complex with Rh in poor solubility easily and precipitates out of the reaction solution.
The nitrogen-containing compound N-methylimidazole as disclosed in European Patent Publication No. 0153834 also forms a complex with Rh in poor solubility easily and precipitates from the reaction solution of methanol carbonylation. In another U.S. Pat. No. 5,442,107, six kinds of heterocyclic nitrogen compounds are selected as the catalyst stabilizer for methanol carbonylation at lower water content:
(1) 2-ethyl-4-methylimidazole,
(2) 4-methylpyridine,
(3) 4-t-butyl-pyridine,
(4) 2-hydroxylpyridine,
(5) 3-hydroxylpyridine,
(6) 4-hydroxylpyridine.
However, at lower water content the influence of the additive as used therein in regard to the reaction rate is not disclosed in the whole patent. It is also disclosed in the same patent the pyridine that is wholly without the substitution of alkyl group will form a complex with Rh in poor solubility easily and will precipitate from the reaction solution of methanol carbonylation at lower water content. This circumstance is similar to what happens to the organic compounds of picoline and N-methylimidazole, as disclosed in the prior arts. When summarizing the results as mentioned above, it is suggested that a hydroxyl group or t-butyl group on the pyridine will apparently benefit the alleviation of the rhodium catalyst precipitation in the reaction solution of methanol carbonylation at lower water content. In contrast, without any substitution or with a methyl group thereon will not apparently alleviate the precipitation of the rhodium catalyst at lower water content. In these prior arts, there is neither description nor suggestion implying pyridine derivative that has substituent other than hydroxyl group or alkyl group has ability to alleviate the precipitation of rhodium catalyst in methanol carbonylation at lower water content.
The prior arts as mentioned above hint several kinds of organic and inorganic salt additives to alleviate or avoid the precipitation of rhodium catalyst when acetic acid is formed by carbonylation of methanol at lower water content. Such kind of technology therefore can save the energy consumed in the distillation process for separation the product acetic acid as well as can reduce additional processing steps such as extraction by solvents and can avoid enlarging some process equipment for the separation.
The present invention provides several kinds of organic additives different from the ones in the prior arts to lower the precipitation of rhodium catalyst when acetic acid is formed by carbonylation of methanol at lower water content. All these catalysts can be employed in either batch process or in a continuous type reactor.
The organic additive as used in the present invention is selected from the following formula (A): 
R1xe2x80x94R5: at least one is xe2x80x94(CH2)mY; m=0xcx9c6; Y is COOH, COOR, SO3H, SO3R; Rxe2x95x90CnH2n+1, n=1-5.
The utilization of nitrogen-containing heterocyclic derivatives having alkyl group or hydroxyl group thereon is disclosed in U.S Pat. No. 5,442,107, however, the ones that have the substituents as shown in said formula (A) are never mentioned.
The present invention relates to an improved process for producing an organic carboxylic acid having (n+1) carbon atoms by reacting an alcohol having n carbon atoms with carbon monoxide in the presence of a rhodium catalyst system. Particularly, the present invention relates to a process for carbonylation of methanol by the catalysis of a rhodium system to synthesize acetic acid. This process comprises directing the alcohol and/or the ester formed by said alcohol and acid along with carbon monoxide to the carbonylation reactor. Said reactor comprises the following components: (1) the rhodium catalyst, (2) the iodide derivative corresponding to the alcohol which is being reacted, (3) the ester formed by acid and alcohol, (4) the carboxylic acid, (5) at least a finite quantity of water, (6) one or more catalyst stabilizers as shown in the following formula (A): 
R1xe2x80x94R5: at least one is xe2x80x94(CH2)mY; m=0xcx9c6; Y is COOH, COOR SO3H, SO3R; Rxe2x95x90CnH2n+1, n=1-5.
It is obvious that even in an extreme test condition with the intention of promoting rhodium precipitation, the organic additive as chosen for the present invention still will not result in a rhodium-containing complex that is partially soluble. Additional advantage of the chosen additive upon comparing with conventional preparation process is it especially effective to prevent the precipitation within the carbonylation reactor at lower water content.
In a preferred embodiment of the above described preparation process, the reacted alcohol carbonylation solution is removed from the carbonylation reactor. Carboxylic acid, rhodium catalyst and catalyst stabilizer are delivered thereafter to a location where the pressure of the carbon monoxide is lower than the carbonylation reactor, and where the separation of the carboxylic acid from other components is also exemplified. Afterwards the rhodium catalyst and the stabilizer are recycled to the carbonylation reactor. In this preferred embodiment, the characteristic of such a separation and recycle process is the coexistence of the catalyst and the stabilizer when carbon monoxide is relatively deficient.
In regard to the water content in the reactor, the present invention is even suitable for the conventional carbonylation plant for methanol, namely 14xcx9c15 wt %. However, this technology is particularly suitable for a carbonylation reactor having water content lower than the conventional method, namely 0.5-12 wt %, most preferably 1-10 wt %.
Upon concerning the alcohol having n carbon atoms, generally it can be any alcohol having 1-20 carbon atoms and having at least one hydroxyl group. More preferably, it is a mono-functional aliphatic alcohol having 1-8 carbon atoms. Most preferably, the alcohol is methanol, ethanol, or propanol, wherein methanol should be the most important material because it has been practiced for commercial use.
The whole stoichiometric equation can be represented as shown below:
ROH+COxe2x86x92RCOOH
Wherein R meets the definition of the organic chain moiety as defined in the previous paragraph. The species of the carboxylic acid product can be easily deduced by alcohol that is being reacted. Taking methanol (Rxe2x95x90CH3) and ethanol (Rxe2x95x90C2H5) for examples, the products are acetic acid and propionic acid, respectively.
The preparation process for the present invention can be operated in a batch process, however, it is desirably in a continuous type for most circumstances. When operated continuously, the alcohol and/or the ester formed by said alcohol and the product carboxylic acid, carbon monoxide, sufficient water to maintained at least a finite concentration of water in the carbonylation reactor, the rhodium catalyst along with the iodide derivative and the catalyst stabilizer are all introduced into the carbonylation reactor. The last four components as mentioned above will not be consumed during the reaction but will be successively recycled back to the reactor from the product outlet, sometimes only minor amount may have to be replenished if in need. Corresponding to the materials successively being introduced into the carbonylation reactor, the liquid product drawn from the carbonylation reactor comprises the product acetic acid, rhodium catalyst, iodide derivatives and the catalyst stabilizer. The net effect is the carbonylation reactor reaches a stable status, and the liquid reaction medium as maintained with stable status comprises water in specified amount, stabilizer, iodide derivative, ester formed by alcohol and carboxylic acid, rhodium catalyst and carboxylic acid. In reality, the esterification between the alcohol and acid is very fast and hence only minor amount of free alcohol will be remained in the carbonylation reactor.
For the preparation process of the present invention, the amount of each component in liquid reaction medium with stable status is preferably at a range as defined below:
Taking carbonylation of methanol to acetic acid for example, the preferred composition of the components is water (1-10 wt %), methyl acetate (0.1-4 wt %), methyl iodide (10-16 wt %), catalyst stabilizer (0.5-30 moles/mole Rh), rhodium catalyst (300-1200 ppm) along with acetic acid.
The carbonylation reactor can be suitably maintained at a temperature of 100-200xc2x0 C., and when the temperature is higher, the reaction rate is faster. The temperature is preferably at a range of 140-200xc2x0 C. The pressure of carbon monoxide is maintained at 10-200 atm, more preferably at 10-100 atm.